Electromagnetic data observed in geophysical experiments, in general, reflect two phenomena: 1) electromagnetic induction (EMI) in the earth, and 2) induced polarization (IP) effects related to the relaxation of polarized charges in rock formations. The IP effect is caused by complex physical-chemical polarization process that accompanies current flow in the earth. These reactions take place in a heterogeneous medium which is most often rock formations in areas of mineralization.
The effective conductivity of rocks is not necessarily a constant and real number but may vary with frequency and be complex. There are several offered explanations for these properties of effective conductivity. Most often they are explained by physical-chemical polarization effects of mineralized particles of the rock material, and/or by the electrokinetic effects in the poroses of reservoirs. This phenomenon is usually explained as a surface polarization of the mineralized particles and the surface of the moisture-porous space, which occurs under the influence of the external electromagnetic field. This influence is manifested by accumulating electric charges on the surface of different grains forming the rock.
The IP effect may also be used to separate the responses of economic polarized targets from other anomalies. This idea was originally discussed in U.S. Pat. No. 3,967,190 issued to Zonge. However, until recently this idea had very limited practical applications because of difficulties in recovering induced polarization parameters from the observed electromagnetic (EM) data, especially in the case of 3-D interpretation required for efficient exploration of the mining targets.
As such, there remains a need for improved methods and technology for remote subsurface formation characterization and mineral discrimination. A new technique is therefore needed and continues to be sought through ongoing research and development efforts.